Wireless power transfer (WPT) or wireless energy transmission is the transmission of electrical power from a power source to a receiving device without using solid wires or conductors. Generally, the term refers to a number of different power transmission technologies that use time-varying electromagnetic fields. In wireless power transfer, a transmitter device is connected to a power source, such as the mains power line, and transmits power by electromagnetic fields across an intervening space to one or more receiver devices, where it is converted back to electric power and utilized. Wireless power transfer techniques may fall into two categories, non-radiative and radiative.
In near-field or non-radiative techniques, power may be transferred over short distances by magnetic fields using inductive coupling between coils of wire or by electric fields using capacitive coupling between electrodes, for example. Using these techniques, it may be possible to transfer power wirelessly within two meters distance with an efficiency of 70% at a frequency of 100 kHz. Applications of this type include, but are not limited to, cell phones, tablets, laptops, electric toothbrush chargers, RFID tags, smartcards, and chargers for implantable medical devices like artificial cardiac pacemakers, and inductive powering or charging of electric vehicles like cars, trains or buses. In radiative or far-field techniques, sometimes called “power beaming”, power may be transmitted by beams of electromagnetic radiation, like microwave or laser beams. These techniques can transport energy longer distances but are typically aimed at the receiver.
Numerous standardized technologies have been developed over the years, including, but not limited to, Alliance for Wireless Power (“A4WP”) which is based on an interface standard (“Rezence”) for wireless electrical power transfer based on the principles of magnetic resonance, where a single power transmitter unit (PTU) is capable of charging one or more power receiver units (PRUs). The interface standard supports power transfer up to 50 Watts, at distances up to five centimeters. The power transmission frequency may be 6.78 MHz, and up to eight devices can be powered from a single PTU depending on transmitter and receiver geometry and power levels. A Bluetooth Smart link may be provided in an A4WP system for controlling power levels, identification of valid loads, and protection of non-compliant devices.
Another standardized WPT technology includes “Qi” which relies on electromagnetic induction between planar coils. A Qi system may include a base station, which is connected to a power source and provides inductive power, and a device that consumes inductive power. The base station may include a power transmitter having a transmitting coil that generates an oscillating magnetic field; the device may include a power receiver holding a receiving coil. The magnetic field from the transmitter may induce an alternating current in the receiving coil by Faraday's law of induction. A further standardized WPT technology includes “Powermat” adopted by the Power Matters Alliance (PMA), which is based upon inductively coupled power transfer, where a transmitter may vary current in a primary induction coil to generate an alternating magnetic field from within a charging spot. The receiver in the consuming device may have its own induction coil that takes power from the magnetic field and converts it back into electrical current to charge the device battery. An additional part of the technology is the use of system control communication via Data over Coil (DoC), where the receiver may send feedback to the transmitter by changing the load seen by the transmitter coil. The protocol is frequency based signaling, thus enabling fast response of the transmitter.
WPT systems often require adaptive tuning in order to modify resonance for a transmitter and/or receiver. Using adaptive tuning, the impedance of transmitter and/or receiver circuits may be modified to match one another (impedance matching). However, impedance matching often involves using a DC bias from an isolated switching mode power supply (SMPS) that includes a power transformer, which adds more size, weight and cost to the system and reduces design flexibility. Furthermore, SMPS-related electromagnetic interference (EMI) may cause issues in WPT circuitry and in some cases may lead to system failure. Accordingly, technologies are needed to provide efficient adaptive tuning without requiring the use of power transformers.